Multifunctional Nanolabels Based on Polydopamine Nanospheres for Sensitive Alpha Fetoprotein Electrochemical Detection

Multifunctional nanolabels with both facile signal amplification and high selectivity are essential for the sensitive determination of the vital tumor marker alpha fetoprotein (AFP). Here, we prepared an immunosensing nanolabel (Ab2-PDA-AF) for convenient AFP detection by grafting the detection antibody (Ab2) and the electroactive redox probe aminoferrocene (AF) onto polydopamine (PDA) nanospheres through Michael addition and Schiff base reactions. The electrochemical signal is dually amplified by the Ab2-PDA-AF nanolabels when the nanolabels are selectively bound to the electrode. The electrochemical signal can be first amplified by oxidizing AF of the Ab2-PDA-AF on the electrode. The signal can be further amplified by the redox reaction of PDA nanospheres with the linked AF/AF+ on the nanospheres. More importantly, the integration of AF-PDA-Ab2 favors a quick electron transfer between AF and PDA nanospheres. Redox probes of AF/AF+ are bonded to the nanolabels instead of dispersing in an electrolyte solution, thus decreasing the electron-transfer resistance and increasing the current signal. The interfacial kinetic analysis demonstrates that the charge-transfer resistance is decreased by 80% by using Ab2-PDA-AF nanolabels. The sensor displays a low detection limit of ca. 0.01 pg/mL, as well as excellent selectivity, reproducibility, and stability. The linear calibration plot of the AFP immunosensor has been obtained in a range of 0.5–1000.0 pg/mL. The capability of the immunosensor has been verified by implementing the detection of AFP in human serum samples.